aterial which passes the 100-mesh sieve
should be used. It will be ground still finer in the process. For the
final polishing, a little infusorial earth or even kaolin will do.

The surface to be ground is moistened with water and dusted over with a
little of the abrasive. The plug is now inserted in the stopcock, and
turned with a gentle pressure. This turning should be in the same
direction for several revolutions, then in the opposite direction for
several more revolutions, etc. As the abrasive becomes finer during the
grinding, a little more may be added if necessary. In general, only a
little grinding will be required, and one small pinch of carborundum or
emery will be ample. The beginner usually grinds too much, and with too
coarse material. As the grinding surface becomes dry, water is added
drop by drop, and the grinding continued until the abrasive seems to be
reduced to an impalpable powder, most of which has been squeezed out of
the stopcock. The two surfaces in the stopcock are usually grinding upon
each other at this stage, and inspection will show whether the contact
between them is uniformly good. If not, the grinding must be continued
with a little fresh abrasive. If contact appears to be good, the
surfaces are ground together for a little with practically no abrasive,
so as to polish them, and the joint is then washed out and tested.

In grinding in a new plug to replace a broken one, the plug selected
should have practically the same taper as the seat into which it is to
be ground, and should be a very little too large. Care must be taken to
so distribute the abrasive material as to grind mostly on the places
where the plug fits tightly.

=Sealing on a New Tube.=--It frequently happens that one of the tubes of
the stopcock is broken off close to the cock itself, and a new one must
be joined to the stub of the old one. With care, this may often be
successfully done even where the break is within 1/4 inch of the
stopcock. The first step is to clean and dry the stopcock, remove the
plug, cork the open ends of the stopcock sleeve and the other tube, and
wind a couple of layers of asbestos cord carefully over the sleeve and
the most of the corks which close it. A suitable tube, having as near as
possible the same diameter and wall strength as the one broken off, is
selected and a piece the desired length cut off. The broken end of the
tube on the stopcock is now squared off as well as possible, by cutting